The present invention is related to strands and bundles of filaments separated by resin containing heat expandable particles applied during the strand-forming operation or subsequently after the strand has been formed. Strands of manufactured filaments of reinforcing materials are usually made of monofilaments drawn from a feeder and gathered into an untwisted bundle which later can be used for reinforcement of composite structures such as glass fiber/resin laminates, pultruded, filament wound, compression molded, or other composite structures. Such strands and bundles can also be twisted into yarns adaptable to being woven into textile fabrics. The invention has application to use in textile fiber structures of a wide spectrum of materials including glass, carbon, polyethyene, nylon, aramid as well as metallic filaments such as of aluminum and steel.
Strands of such monofilaments during formation are usually gathered together over a gathering device in which the filaments are compacted, densifying the strands much more so than is experienced with natural fiber textile strands. In being compacted, the density of the strand approaches the density of the base material of which the filaments are made. The advantage of such strand product as a reinforcing structure is usually the high tensile strength of the monofilaments. A disadvantage of the concentration of filaments is the reduced capability of the strand or bundle to distribute the strength of the filaments in the product in which it is incorporated as a reinforcement. In order to distribute the strength, more strands are often used than are needed for the design strength requirement. For example, in pultrusion, the strands are needed to fill the die and to pull a mass of matrix material through the forming die. If the monofilaments could be spread out, they would serve the latter function but fewer strands would be needed, thereby providing lower finished product density and also an economic advantage, while still providing the desired tensile strength in the product. Such techniques have been attempted in the past by way of using spun roving and by texturizing the strand or bundle to bulk the reinforcement, in addition to using mats. The present invention, however, provides considerably more spacing between filaments in such filament structures than has been possible with much of the prior art.